This invention relates to compressive load carrying bearings and more particularly to laminated bearings of the type comprising alternating bonded layers of a substantially resilient material, such as an elastomer, and a substantially non-extensible material, such as a metal.
It is well known that the compressive load carrying ability of a given thickness of an elastomeric material may be increased many times by subdividing it into a plurality of layers and separating the layers by intervening layers of a non-extensible material. At the same time, however, the ability of the resilient material to yield in shear in a direction parallel to the layers is substantially unaffected. This concept has been adopted or utilized in the design of different forms of laminated bearings, as exemplified by the following: application Ser. No. 061,009 filed on July 26, 1979, now U.S. Pat. No. 4,291,925; application Ser. No. 067,993 filed on Aug. 20, 1979, now U.S. Pat. No. 4,256,354; and application Ser. No. 083,598 filed on Oct. 11, 1979, now U.S. Pat. No. 4,286,827, and the prior art cited therein. This concept has also been adopted or utilized in the design of different forms of flexible couplings, such as those described in application Ser. No. 185,028 filed on Sept. 8, 1980; and application Ser. No. 233,711 filed on Feb. 12, 1981, and the prior art cited therein. The listed cases are all commonly assigned to the assignee of the present invention.
Laminated elastomeric bearings of various types are commonly used in commercial applications where it is necessary to carry large, compressive loads in a first direction and also to accommodate limited relative movement in, for example, several other directions. The bearings are designed so that the large, compressive loads are carried generally perpendicular to the resilient lamillae. For the usual laminated bearing application it is desirable, if not essential, to have a bearing design which provides an optimum combination of load-carrying capability, spring rate, and strain distribution consistent with cost and life-expectancy considerations. For example, a bearing of conical geometry employed in a helicopter main rotor retention system is required to undergo dynamic and static torsional deflection, including cyclic motion, as well as dynamic and static compressive loading. The bearing experiences shear strain produced from a torsional deflection about the bearing central axis. Additionally shear strain is induced by application of the compressive loads which may be either axially or radially directed. Thus, torsional shear strain and compression-induced shear strain are present in many operating situations.
The bearing cases listed above describe various means, for example, for improving strain distribution in the bearings by controlling the modulus of elasticity from layer to layer or within a layer so as to improve operation in normal applications of the bearing. The present invention concerns another property or characteristic of elastomer and attempts to resolve a problem which has heretofore limited the practical temperature operating range of the bearing.
Experience with such laminated devices has recently shown that the operational characteristics of the device are less than desirable when it is used in low-temperature environments, for example, when the ambient temperature is below 0.degree. F.
A problem with such devices in such environments is that heat generated by hysteresis produces a non-uniform temperature gradient across the laminate due to the "heat sink" effect attributable to the inner and outer housings, the air, and the metal layers in the laminate. Since the shear spring rate and modulus of the elastomer vary with temperature, the non-uniform temperature gradient results in a non-uniform strain distribution within the bearing which can cause premature degradation of elastomer layers after prolonged operation in the environment. Further, the operational characteristics of the device are adversely affected since certain layers adjacent the heat sinks are sharing a lower percentage of the total torsional motion than possibly the designer of the device had anticipated.
It is therefore an object of the present invention to provide a laminated device suited to cold weather operation which is of a design further suited to the economical manufacture thereof.
It is a further object of the present invention to provide a more uniform temperature distribution over each elastomeric layer and from layer to layer within the laminate of a device, such as a bearing or coupling, so as to improve the operating characteristics of the device for a low ambient temperature use.
Another object of the present invention is to provide a more uniform strain distribution than would otherwise occur due to the non-uniformity of the temperature gradient in cold weather use of such a device.
These and other objects of the invention are addressed by providing a device, such as a laminated elastomeric bearing or coupling comprising a substantially rigid interior housing and exterior housing, said housings joined together by bonded concentric lamillae comprising alternating strata of resilient material and substantially non-extensible material characterized by having a resilient layer adjacent the inner and outer housings more highly damped than the other resilient layers within the laminate. Further, the resilient layers within said device can be segmented into a plurality of discrete portions, or otherwise graduated, so as to permit more highly damped elastomer material to be selectively disposed adjacent the convection boundary between, for example, the laminate and the ambient air than is disposed away from said boundary. Thus, the laminate has more highly damped elastomer material about its periphery than that disposed away from said periphery and within the laminate. By this expediency the operation of the device in low temperature applications is improved.